WASHINGTON — During a period of universal warming 11 billion years ago, quasars — the brilliant core of active galaxies — produced fierce radiation blasts that stunted the growth of some dwarf galaxies for approximately 500 million years.

This important conclusion comes from a team of astronomers that used the new capabilities of NASA's Hubble Space Telescope to probe the invisible, remote universe. The team's results will be published in the October 20 issue of The Astrophysical Journal.

Using Hubble's Cosmic Origins Spectrograph (COS), the astronomers identified this era, from 11.7 to 11.3 billion years ago, when the ultraviolet light emitted by active galaxies stripped electrons off helium atoms. The process, known as ionization, heated the intergalactic helium from 18,000 degrees Fahrenheit to nearly 40,000 degrees. This inhibited the gas from gravitationally collapsing to form new generations of stars in some small galaxies.

Because of its greatly improved sensitivity and lower background "noise" compared to previous spectrographs in space, the COS observations were ground-breaking. The observations allowed scientists to produce more detailed measurements of the intergalactic helium than previously possible.

"These COS results yield new insight into an important phase in the history of our universe," said Hubble Program Scientist Eric Smith at NASA Headquarters in Washington.

Michael Shull of the University of Colorado in Boulder and his team studied the spectrum of ultraviolet light produced by a quasar and found signs of ionized helium. This beacon, like a headlight shining through fog, travels through interspersed clouds of otherwise invisible gas and allows for a core sample of the gas clouds.

The universe went through an initial heat wave more than 13 billion years ago when energy from early massive stars ionized cold interstellar hydrogen from the big bang. This epoch is called reionization, because the hydrogen nuclei originally were in an ionized state shortly after the big bang.

The Hubble team found it would take another two billion years before the universe produced sources of ultraviolet radiation with enough energy to reionize the primordial helium that also was cooked up in the big bang. This radiation didn't come from stars, but rather from super massive black holes. The black holes furiously converted some of the gravitational energy of this mass to powerful ultraviolet radiation that blazed out of these active galaxies.

The helium's reionization occurred at a transitional time in the universe's history when galaxies collided to ignite quasars. After the helium was reionized, intergalactic gas again cooled down and dwarf galaxies could resume normal assembly.

"I imagine quite a few more dwarf galaxies may have formed if helium reionization had not taken place," Shull said.

So far, Shull and his team only have one perspective to measure the helium transition to its ionized state. However, the COS science team plans to use Hubble to look in other directions to determine if helium reionization uniformly took place across the universe.